The present invention relates to a filter module constructed as a demultiplexing/multiplexing module suitable for an optical communication system based on the wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) transmission modes.
In general, in conventional optical communication systems, a large number of demultiplexing modules are used to demultiplex (split) demultiplexing (splitting) wavelength multiplex signals transmitted through optical fiber into the individual wavelength signals and a large number of multiplexing modules are used to multiplex (combine) a plurality of optical signals different in wavelength into an optical fiber (for example, National Publication of International Patent Application No. 10-511476).
Additionally, three-port filter modules as shown in FIG. 10 have been conventionally used as filter modules capable of demultiplexing/multiplexing of an optical signal in a particular wavelength band.
As shown in FIG. 10, such a filter module comprises a single optical fiber collimator (hereinafter referred to as “single FC”) 71 and a dual optical fiber collimator (hereinafter referred to as “dual FC”) 72, and a cylindrical tube 73 for integrally holding both FC's 71, 72.
The single FC 71 comprises a single-core capillary 76 for holding an optical fiber 75, a refractive index distribution type rod lens 77, and a tube 78 for integrally holding these components. The two opposing faces of the single-core capillary 76 and rod lens 77 are each polished so as to be inclined. The single-core capillary 76 and the rod lens 77 are subjected to core alignment for the purpose of preventing the inclination deviation, and are fixed in the tube 78 with the aid of an adhesive so that the core aligned arrangement may be maintained.
On the other hand, the dual FC 72 comprises a two-core capillary 82 for integrally holding two optical fibers 80, 81 and a refractive index distribution type rod lens 83. The two opposing faces of the two-core capillary 82 and rod lens 83 are each polished so as to be inclined. The two-core capillary 82 and the rod lens 83 are subjected to core alignment and are jointed to each other, with the aid of the rings of an adhesive 85 applied onto the peripheries of the two opposing end faces, for the purpose of maintaining the core aligned arrangement. Additionally, the jointed parts are bonded to each other with the aid of an adhesive 86 for reinforcement covering the jointed parts. Yet additionally, an inserted short tube 84 is made to fit around the two-core capillary 82.
After both FC's 71, 72 have been fabricated, a board with a wavelength selective transmitting film (filter) 87 is bonded onto the front face of the dual FC 72, namely, the end face of the rod lens 83 with the aid of an adhesive 90 in a core aligned manner. Additionally, the bonded parts are bonded with the aid of an adhesive 91 for reinforcement covering the bonded parts.
Subsequently, both FC's 71, 72 are subjected to a mutual core alignment, both FC's are bonded in the interior of the tube 73 with the aid of an adhesive 92 for the purpose of maintaining the core aligned arrangement.
The position of both open ends of the tube 73 are respectively made to coincide with the positions of the end faces of both FC's 71, 72. Additionally, a resin based adhesive 88 is applied onto the respective end faces for the purpose of holding the optical fiber 75 and the optical fibers 81, 80. Thus, the filter module shown in FIG. 10 is fabricated.
According to the filter module constituted as described above, a wavelength multiplexed optical signal is input into or output from one of the two optical fibers 80, 81 of the dual FC 72 (for example, the optical fiber 80), and an optical signal of the wavelength band reflected on the filter 87 is input into or output from the other optical fiber (for example, the optical fiber 81). Additionally, an optical signal in the wavelength band selectively passing through the filter 87 is input into or output from the optical fiber 75 provided in the single FC 71.
In this connection, the conventional filter module has the following problems.
(1) The size of the filter module is large. Additionally, the three ports are arranged apart at both ends of the module. Accordingly, when a plurality of filter modules are connected in cascade to form a multichannel demultiplexing/multiplexing unit, a large space is needed for wiring the individual filter modules and optical fibers for connecting the filter modules. More specifically, when wiring is made by bending the optical fibers, a large space is needed, which permits ensuring the curvature radii of the fibers sufficiently large (for example, 30 mm or more) to prevent the break damage of the optical fibers. Thus, the size of the fabricated multichannel demultiplexing/multiplexing unit becomes large, and it is difficult to reduce the size thereof.
(2) The number of components is large, which leads to the increase in fabrication cost.
(3) The number of the processes for core alignment and fixation is large; more specifically, the following processes (a) to (c) for core alignment and fixation are needed.
(a) A process, in the fabrication of a single FC 71, for core alignment of the optical fiber 75 and the rod lens 77, and fixation thereof.
(b) A process, in the fabrication of a dual FC 72, for core alignment of the two-core capillary 82 and the rod lens 83, and fixation thereof.
(c) A process for core alignment of the single FC 71 and the dual FC 72, and fixation of both in the tube 73 when both FC's 71, 72 are fixed in the tube 73.
(d) A process for core alignment and fixation of the filter 87 to the rod lens 83 when the filter 87 is bonded onto the end face of the rod lens 83.
As described above, three or four processes for core alignment and fixation are needed, which results in elongation of the fabrication time, increase of the fabrication cost, and reliability degradation of the product characteristics of the filter module.